Attention-deficit hyperactivity disorder (ADHD) affects 3-5% of elementary school age children and is characterized by significant attentional weakness, impulsivity, and hyperactivity.1 However, since mental retardation has generally been used as an exclusionary criteria in most studies of ADHD children, there are few guidelines for determining a diagnosis of ADHD within this group and limited information on the efficacy of a stimulant drug therapy as a primary treatment modality. Consequently, it is often necessary to rely heavily upon the research focusing on non- retarded children to make decisions regarding diagnosis and clinical management in this population. The goals of the present proposal are to increase our knowledge of ADHD among preschool and school age children who function within the educable mentally retarded (EMR) range. This group comprises 80% of the mentally retarded population with estimates that of up to 7.5% of EMR children in the public schools and 5.5% of developmentally delayed preschoolers are prescribed stimulant medication.2 Descriptive issues will be addressed by contrasting ADHD/EMR children with matched non- ADHD/EMR children on a range of behavioral, attentionsl, learning, and social measures. Developmental issues will be examined by contrasting ADHD/EMR preschoolers and school age children on these same measures. A longitudinal study will be conducted to determine the predictive validity of early identification of behavioral deficits among EMR preschoolers in relationship to diagnosis and outcome at school age. Finally, treatment issues will be addressed by examination of the efficacy of stimulant mediation on a similar set of measures with school age ADHD/EMR children. A double-blind medication trial comparing two doses of Ritalin (0.3 and 0.6 mg/kg) and a placebo will be used. Information obtained form this study will offer important knowledge regarding description of this population and the efficacy of stimulant medication.